Thermal evaluation of a Cs-loaded waste vitrification

Danilo Lopes Costa e Silva; Mariana Silva Araújo; Denise Alves Fungaro; Sonia Mello-Castanho

doi:10.15392/2319-0612.2024.2646

Authors

Danilo Lopes Costa e Silva Instituto de Pesquias Energéticas e Nucleares
Mariana Araújo Instituto de Pesquisas Energéticas e Nucleares https://orcid.org/0000-0002-0519-6263
Denise Fungaro Instituto de Pesquisas Energéticas e Nucleares https://orcid.org/0000-0003-1618-0264
Sonia Mello Instituto de Pesquisas Energéticas e Nucleares https://orcid.org/0000-0002-0155-9100

DOI:

https://doi.org/10.15392/2319-0612.2024.2646

Keywords:

cesium-137, nuclear waste vitrification, niobium-modified glass, zeolite A

Abstract

Nuclear power generation has been increasing worldwide over the years, helping to avoid the emission of billions of tons of carbon dioxide (CO₂) compared to coal power generation, making it a reliable method for providing green energy. However, as a byproduct of nuclear reactors, as well as fuel processing plants, hospitals, and research institutes, radioactive waste is generated. These wastes pose a significant risk to human health and the environment due to the penetration of radioactivity into tissues, which damages DNA. Therefore, they need to be managed appropriately before long-term disposal in geological repositories. Among the many radionuclides found in high-activity waste, ¹³⁷Cs is of particular concern due to its high mobility in water systems, requiring special methods for its capture and immobilization in stable matrices. In this study, the immobilization of ¹³⁷Cs radionuclides in a borosilicate glass doped with niobium (Nb) was thermally evaluated through a vitrification process, using a synthetic type A zeolite saturated with ¹³³Cs (stable) as a simulated radioactive waste. The incorporation of 40.0 wt.% of this material into the glass composition resulted in a vitrified waste with good melting homogeneity and thermal stability. Several changes due to the incorporation were detected in thermal analysis (DTA), with events such as glass transition, initial crystallization, complete crystallization, and subsequent melting all shifting to higher temperatures. The compositional changes induced by vitrification moved the system to new locations in the ternary equilibrium phase diagrams of the subsystems, in different compatibility triangles, and closer to higher liquidus temperatures than those observed for the raw glass matrix. Through crystallization induced by heat treatment, crystalline phases were obtained, as indicated in the phase diagrams. Cs atoms previously immobilized in the glass network structure became components of pollucite crystals (CsAlSi₂O₆) during heating up to 800 ºC. These results are promising for using this glass composition to immobilize waste containing ¹³⁷Cs, as Cs atoms showed excellent interaction with this system in both the glass and crystalline phases.

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